1. Field of the Invention
The invention relates to a control apparatus and a control method for a negative pressure generating apparatus. More specifically, the invention relates to a control apparatus and a control method for a negative pressure generating apparatus, which prevent or suppress occurrence of clogging due to freezing in the negative pressure generating apparatus.
2. Description of the Related Art
In a vehicle, an ejector is used to supply a negative pressure whose magnitude is larger than that of a negative pressure to be taken from an intake passage (for example, an intake manifold or a surge tank) in an intake system for an internal combustion engine, to a negative pressure operating device such as a brake booster (hereinafter, the negative pressure to be taken from the intake passage may be referred to as “intake pipe negative pressure”). The ejector is described, for example, in Japanese Patent Application Publication No. 2005-69175 (JP-A-2005-69175), Japanese Patent Application Publication No. 2005-297654 (JP-A-2005-297654), and Japanese Patent Application Publication No. 2006-131003 (JP-A-2006-131003).
For example, the publication No. 2005-69175 describes a technology in which the ejector is prohibited from being operated until a predetermined time elapses after the engine is started. Because the temperature of a coolant for the engine is increased when the predetermined time elapses after the engine is started, it is possible to prevent or suppress occurrence of clogging due to freezing in the negative pressure generating apparatus to some extent, according to the technology described in the publication No. 2005-69175. However, because intake air flowing in the ejector is air taken from outside, for example, when the engine is brought from a cold state (in which, for example, the temperature of the coolant is lower than 75° C.) to a warm state after the engine is started (in other words, when warming-up is completed), the temperature of air flowing in the ejector is hardly changed. Accordingly, in the technology described in the publication No. 2005-69175, for example, when an outside air temperature is equal to or lower than 0° C., it is not possible to sufficiently prevent or suppress occurrence of clogging due to freezing.
Further, because the ejector generates the negative pressure whose magnitude is larger than that of the intake pipe negative pressure using the venturi effect, an inner passage where the negative pressure is generated is narrowed in the ejector. Therefore, when the intake air flows in the inner passage, the temperature of the intake air is decreased. Thus, when the temperature of the intake air is low, and is decreased as described above, clogging may occur due to freezing of moisture and the like contained in the intake air even after warming-up is completed, partly because the temperature in an engine room is still low. That is, clogging due to freezing may occur in the entire negative pressure generating apparatus when the outside air temperature is equal to or lower than 0° C. In addition, even if the outside air temperature is higher than 0° C., clogging due to freezing may occur in the ejector and the negative pressure generating apparatus including the ejector when the temperature of the intake air is decreased as described above. If clogging due to freezing occurs, it may not be possible to use the ejector for a long period until the clogging due to freezing is eliminated.